The present invention relates generally to a method and apparatus for balancing an object. More particularly, the invention relates to a method and apparatus in which effects due to both static and dynamic imbalance are excited by a small angular oscillation rather than by spin as in ccnventional balancers.
Conventional balancing machines excite effects due to static and dynamic imbalance in an object by spinning it and detecting those effects by various means. Typically, the balancer spins the object about a vertical axis and deduces the imbalance by detecting the resulting horizontal reaction forces and torques. By knowing these forces, and torques, one can readily determine the adjustments needed to correct the balance of the balance.
Because the reactions due to imbalance are proportional to the square of the spin rate, the signal-to-noise ratio tends to be low at low spin rates. Accordingly, it is usually desirable to maintain a fairly high spin rate to reduce the effects of the measurement noises. This works quite well when the object being tested is relatively smooth and symmetrical; but on irregular-shaped objects, such as on the new breed of shuttle-launched spacecraft with their unusual angular shapes and oddly placed solar cell paddles, aerodynamic forces cause effects which are indistinguishable from those caused by the imbalance and which cannot be eliminated by simply increasing the rate of spin. This is because windage noise is also proportional to the square of the spin rate; and as the spin rate is increased, these oddly shaped objects create so much wind noise that the precision of the balancing process is totally destroyed.
In order to overcome this serious problem, one practice utilized in the prior art has been to perform the balancing operations inside a vacuum chamber. Although this does circumvent the problem, it also significantly increases the cost of the balancing process. The balancing could also be done with the paddles removed from the spacecraft to reduce the effects due to aerodynamic forces, or the balancer could be placed within a spinning shroud where the surrounding air will move with the spacecraft. If desired, the balancing could even be done after the spacecraft is in orbit. All of these approaches, however, have a number of drawbacks, some of which are readily apparent, others of which are more obscure. What is really needed is a balancing machine that can balance oddly shaped objects in an ordinary room without significant aerodynamic interference.